GSGRAPE,
I have had G/V installed on 2008 F450/6.4LDiesel/4.88 rear for 5 years. When it works, it's a dream!! I calculated about 30% improvement in MPG with mine engaged. The only thing I would caution about is towing with the G/V engaged. When I bought mine, G/V said it is good for towing up to 25K GCWR, which was fine for my setup. After about 20K miles the G/V failed and it was replaced, while I was on a road trip with the RV. G/V sent replacement to a Billings, MT dealer who installed the rebuilt replacement. Then, they said not to tow with truck in Overdrive (I always towed with the Tow/Haul mode on) - and subsequently you can now find that warning on their website (which wasn't there in '08/or in their owner's manual). After the warranty replacement unit went in it failed after about 15K miles. I haven't replaced it yet - good thing about the G/V is that if it's broken, you're not disabled. Apparently, F450's just like to eat these G/V's up. My driving style is very conservative on the highway, so I wish I could say it got worked too hard. This was my experience, for what it's worth.
Good luck, and enjoy your Heartland!!